The discussion of prior art information is herein divided into three parts in this Background section: (a) a discussion on cathode active materials for lithium secondary batteries (including lithium metal batteries and lithium-ion batteries) and long-standing issues associated with these materials; (b) the new 2-D nano material called “graphene” and its prior use as a conductive substrate material for the cathode active material; and (c) graphene-based foamed material called “graphene foam”.
Historically, today's most favorite rechargeable energy storage devices—lithium-ion batteries—actually evolved from rechargeable “lithium metal batteries” that use lithium (Li) metal as the anode and a Li intercalation compound (e.g. MoS2) as the cathode. Li metal is an ideal anode material due to its light weight (the lightest metal), high electronegativity (−3.04 V vs. the standard hydrogen electrode), and high theoretical capacity (3,860 mAh/g). Based on these outstanding properties, lithium metal batteries were proposed 40 years ago as an ideal system for high energy-density applications.
Due to some safety concerns of pure lithium metal, graphite was implemented as an anode active material in place of the lithium metal to produce the current lithium-ion batteries. The past two decades have witnessed a continuous improvement in Li-ion batteries in terms of energy density, rate capability, and safety. However, the use of graphite-based anodes in Li-ion batteries has several significant drawbacks: low specific capacity (theoretical capacity of 372 mAh/g as opposed to 3,860 mAh/g for Li metal), long Li intercalation time (e.g. low solid-state diffusion coefficients of Li in and out of graphite and inorganic oxide particles) requiring long recharge times (e.g. 7 hours for electric vehicle batteries), inability to deliver high pulse power (power density<<1 kW/kg), and necessity to use pre-lithiated cathodes (e.g. lithium cobalt oxide, as opposed to cobalt oxide), thereby limiting the choice of available cathode materials. Further, these commonly used cathode active materials have a relatively low specific capacity (typically <220 mAh/g). These factors have contributed to the two major shortcomings of today's Li-ion batteries—a low energy density (typically 150-220 Wh/kgcell) and low power density (typically <0.5 kW/kg).
Although several high-capacity anode active materials have been found (e.g., Si with a theoretical capacity of 4,200 mAh/g), there has been no corresponding high-capacity cathode material available. To sum it up, battery scientists have been frustrated with the low energy density of lithium-ion cells for over three decades! Current cathode active materials commonly used in Li-ion batteries have the following serious drawbacks:                (1) The practical capacity achievable with current cathode materials (e.g. lithium iron phosphate and lithium transition metal oxides) has been limited to the range of 150-250 mAh/g and, in most cases, less than 200 mAh/g.        (2) The production of these cathode active materials normally has to go through a high-temperature sintering procedure for a long duration of time, a tedious, energy-intensive, and difficult-to-control process.        (3) The insertion and extraction of lithium in and out of these commonly used cathodes rely upon extremely slow solid-state diffusion of Li in solid particles having very low diffusion coefficients (typically 10−8 to 10−14 cm2/s), leading to a very low power density (another long-standing problem of today's lithium-ion batteries).        (4) Current cathode active materials are electrically and thermally insulating, not capable of effectively and efficiently transporting electrons and heat. The low electrical conductivity means high internal resistance and the necessity to add a large amount of conductive additives, effectively reducing the proportion of electrochemically active material in the cathode that already has a low capacity. The low thermal conductivity also implies a higher tendency to undergo thermal runaway, a major safety issue in lithium battery industry.        (5) The most commonly used cathodes, including lithium transition metal oxides, contain a high oxygen content that could assist in accelerating the thermal runaway and provide oxygen for electrolyte oxidation, increasing the danger of explosion or fire hazard. This is a serious problem that has hampered the widespread implementation of electric vehicles.        
Thus, there is a strong and urgent need to develop high-capacity cathode active materials. Metal fluorides with metallic cations in high oxidation states and a strong ionic character of the M-F bonds (M=a metal) have been proposed as alternative cathode active materials due to their high theoretical energy densities. For instance, FeF3 has attracted considerable interests because of its low cost and low toxicity. However, the highly ionic character induces a large band gap, thus metal fluorides have very poor electronic conductivity. In addition, LiF, the product of the conversion reaction, is also highly insulating. Accordingly, metal fluoride electrodes often suffer severely from slow reaction kinetics and low lithium storage capacity, significantly lower than the theoretical capacity.
Several attempts have been made to overcome these issues, but with very limited success. For instance, an effort was made to enhance the electrochemical activity by reducing the metal fluoride particle size to the nanometer range for the purpose of achieving shorter electron-conducting paths and larger reaction surface. In this example, Badway, et al. reported a FeF3/C nanocomposite through ball-milling [F. Badway, et al., “Carbon metal fluoride nanocomposites high-capacity reversible metal fluoride conversion materials as rechargeable positive electrodes for Li batteries,” J. Electrochem. Soc. 150 (2003) A1318-A1327]. However, this method has several drawbacks, including difficulties in controlling materials properties and production of a significant number of defects.
The deposition of FeF3 on conductive carbon particle surfaces without ball milling was proposed as another means of improving electrode performance. For instance, Kim et al. have fabricated carbon nanotube/FeF3 composites by nucleation of FeF3 on the defects of CNT surfaces generated by HF etching [S. W. Kim, et al., “Fabrication of FeF3 nanoflowers on CNT branches and their application to high power lithium rechargeable batteries,” Adv. Mater. 22 (2010) 5260-5264]. This strategy has been followed by others to fabricate FeF3 on activated carbon micro bead [L. Liu, et al., “Synthesis and electrochemical performance of spherical FeF3/ACMB composite as cathode material for lithium-ion batteries,” J. Mater. Sci. 47 (2012) 1819-1824]. Liu et al. proposed a low-temperature in situ approach for the synthesis of uniform FeF3 nano particles on reduced graphene oxide (rGO) sheets suspended in ethanol solution [J. Liu, et al., “Mild and cost-effective synthesis of iron fluoride-graphene nanocomposites for high-rate Li-ion battery cathodes,” J. Mater. Chem. A 1 (2013) 1969-1975]. However, the loading level of FeF3 on rGO and the rate capability of the FeF3/graphene composites remain too low for practical applications.
Other attempts to use graphene as a conductive additive for FeF3 all fall short in providing good rate capability, high energy density, and long cycle life. Examples of these earlier efforts are [X. Zhao, et al., “Photothermal-assisted fabrication of iron fluoride-graphene composite paper cathodes for high-energy lithium-ion batteries,” Chem. Commun. 48 (2012) 9909-9911] and [Q. Chu, et al. “Reduced graphene oxide decorated with FeF3 nanoparticles: Facile synthesis and application as a high capacity cathode material for rechargeable lithium batteries,” Electrochim. Acta. 111 (2013) 80]. Although Q. Chu, et al. claim to achieve a high specific capacity of 476 mAh/g, this capacity is achieved only when the current density is at a practically useless value of 50 mA/g (an excessively low discharge rate). Further, the specific capacity rapidly drops to approximately 110 mAh/g after only 50 charge-discharge cycles (see FIG. 5B of Chu, et al.). Furthermore, this maximum achievable value of 476 mAh/g is significantly lower than the theoretical specific capacity of 712 mAh/g for FeF3, indicating a low active material utilization rate (i.e. a significant proportion of the active material is not fully utilized).
Due to extremely poor electrical conductivity of all cathode active materials in a lithium-ion or lithium metal cell, a conductive additive (e.g. carbon black, fine graphite particles, expanded graphite particles, or their combinations), typically in the amount of 2%-15%, must be added into the electrode. However, the conductive additive is not an electrode active material. The use of a non-active material means that the relative proportion of an electrode active material is reduced or diluted. For instance, the incorporation of 5% by weight of PVDF as a binder and 5% of carbon black as a conductive additive in a cathode would mean that the maximum amount of the cathode active material (e.g., lithium cobalt oxide) is only 90%, effectively reducing the total lithium ion storage capacity. Since the specific capacities of the more commonly used cathode active materials are already very low (140-220 mAh/g), this problem is further aggravated if a significant amount of non-active materials is used to dilute the concentration of the active material.
Carbon black (CB) materials, as a conductive additive, have several drawbacks: (1) CBs are typically available in the form of aggregates of multiple primary particles that are typically spherical in shape. Due to this geometric feature (largest dimension-to-smallest dimension ratio or aspect ratio ˜1) and the notion that CBs are a minority phase dispersed as discrete particles in an electrically insulating matrix (e.g. lithium cobalt oxide and lithium iron phosphate), a large amount of CBs is required to reach a percolation threshold where the CB particles are combined to form a 3-D network of electron-conducting paths. (2) CBs themselves have a relatively low electrical conductivity and, hence, the resulting electrode remains to be of relatively low conductivity even when the percolation threshold is reached. A relatively high proportion of CBs (far beyond the percolation threshold) must be incorporated in the cathode to make the resulting composite electrode reasonably conducting.
Clearly, an urgent need exists for an effective supporting material for metal fluorides and chlorides that enables a high cathode active material utilization rate, high specific capacity at both high and low charge/discharge rates (not just at a low rate), high rate capability, long cycle-life, and improved heat dissipation generated during a battery operation. These are the main objectives of the instant invention.
This supporting or “enabling” material also must be electrically conductive. Preferably, this electrically conductive supporting material is also of high thermal conductivity. Such a thermally conductive additive would be capable of dissipating the heat generated from the electrochemical operation of the Li-ion battery, thereby increasing the reliability of the battery and decreasing the likelihood that the battery will suffer from thermal runaway and rupture. With a high electrical conductivity, there would be no need to add a high proportion of conductive additives.
The present invention goes beyond and above the prior art efforts of using solid graphene sheets, or nano graphene platelets (NGPs), to form a 3-D conductive network to support a cathode active material. Specifically, the instant application makes use of a graphene foam material to protect the cathode active material, by providing several other unexpected functions, in addition to forming a 3-D conducting network. Hence, a brief discussion is herein made on the production of graphene foams and this discussion should be helpful to the reader.
Generally speaking, a foam (or foamed material) is composed of pores and pore walls (the solid portion of a foam material). The pores can be interconnected to form an open-cell foam. A graphene foam is composed of pores and pore walls that contain a graphene material. There are three major methods of producing graphene foams:
The first method is the hydrothermal reduction of graphene oxide hydrogel that typically involves sealing graphene oxide (GO) aqueous suspension in a high-pressure autoclave and heating the GO suspension under a high pressure (tens or hundreds of atm) at a temperature typically in the range of 180-300° C. for an extended period of time (typically 12-36 hours). A useful reference for this method is given here: Y. Xu, et al. “Self-Assembled Graphene Hydrogel via a One-Step Hydrothermal Process,” ACS Nano 2010, 4, 4324-4330. There are several major issues associated with this method: (a) The high pressure requirement makes it an impractical method for industrial-scale production. For one thing, this process cannot be conducted on a continuous basis. (b) It is difficult, if not impossible, to exercise control over the pore size and the porosity level of the resulting porous structure. (c) There is no flexibility in terms of varying the shape and size of the resulting reduced graphene oxide (RGO) material (e.g. it cannot be made into a film shape). (d) The method involves the use of an ultra-low concentration of GO suspended in water (e.g. 2 mg/mL=2 g/L=2 kg/kL). With the removal of non-carbon elements (up to 50%), one can only produce less than 2 kg of graphene material (RGO) per 1000-liter suspension. Furthermore, it is practically impossible to operate a 1000-liter reactor that has to withstand the conditions of a high temperature and a high pressure. Clearly, this is not a scalable process for mass production of porous graphene structures.
The second method is based on a template-assisted catalytic CVD process, which involves CVD deposition of graphene on a sacrificial template (e.g. Ni foam). The graphene material conforms to the shape and dimensions of the Ni foam structure. The Ni foam is then etched away using an etching agent, leaving behind a monolith of graphene skeleton that is essentially an open-cell foam. A useful reference for this method is given here: Zongping Chen, et al., “Three-dimensional flexible and conductive interconnected graphene networks grown by chemical vapour deposition,” Nature Materials, 10 (June 2011) 424-428. There are several problems associated with such a process: (a) the catalytic CVD is intrinsically a very slow, highly energy-intensive, and expensive process; (b) the etching agent is typically a highly undesirable chemical and the resulting Ni-containing etching solution is a source of pollution. It is very difficult and expensive to recover or recycle the dissolved Ni metal from the etchant solution. (c) It is challenging to maintain the shape and dimensions of the graphene foam without damaging the cell walls when the Ni foam is being etched away. The resulting graphene foam is typically very brittle and fragile. (d) The transport of the CVD precursor gas (e.g. hydrocarbon) into the interior of a metal foam can be difficult, resulting in a non-uniform structure, since certain spots inside the sacrificial metal foam may not be accessible to the CVD precursor gas. ( ) This method does not lend itself to embedding anode active material particles therein.
The third method of producing graphene foam also makes use of a sacrificial material (e.g. colloidal polystyrene particles, PS) that is coated with graphene oxide sheets using a self-assembly approach. For instance, Choi, et al. prepared chemically modified graphene (CMG) paper in two steps: fabrication of free-standing PS/CMG films by vacuum filtration of a mixed aqueous colloidal suspension of CMG and PS (2.0 μm PS spheres), followed by removal of PS beads to generate 3D macro-pores. [B. G. Choi, et al., “3D Macroporous Graphene Frameworks for Supercapacitors with High Energy and Power Densities,” ACS Nano, 6 (2012) 4020-4028.] Choi, et al. fabricated well-ordered free-standing PS/CMG paper by filtration, which began with separately preparing a negatively charged CMG colloidal and a positively charged PS suspension. A mixture of CMG colloidal and PS suspension was dispersed in solution under controlled pH (=2), where the two compounds had the same surface charges (zeta potential values of +13±2.4 mV for CMG and +68±5.6 mV for PS). When the pH was raised to 6, CMGs (zeta potential=−29±3.7 mV) and PS spheres (zeta potential=+51±2.5 mV) were assembled due to the electrostatic interactions and hydrophobic characteristics between them, and these were subsequently integrated into PS/CMG composite paper through a filtering process. This method also has several shortcomings: (a) This method requires very tedious chemical treatments of both graphene oxide and PS particles. (b) The removal of PS by toluene also leads to weakened macro-porous structures. (c) Toluene is a highly regulated chemical and must be treated with extreme caution. (d) The pore sizes are typically excessively big (e.g. several μm), too big for many useful applications.
The above discussion clearly indicates that every prior art method or process for producing graphene foams has some major deficiencies. Further, none of the earlier work makes use of graphene foam as a protective material for a cathode active material of a lithium battery.
Thus, it is an object of the present invention to provide a cost-effective process for producing highly conductive, mechanically robust graphene foams in large quantities. This graphene foam also contains cathode active material particles or coating (e.g. transition metal fluoride or chloride) residing in the pores of this foam and being protected by this foam. This process does not involve the use of an environmentally unfriendly chemical. This process enables the flexible design and control of the porosity level and pore sizes.
It is another object of the present invention to provide a process for producing graphene foam-protected cathode active material wherein the graphene foam exhibits a thermal conductivity, electrical conductivity, elastic modulus, and/or compressive strength that is comparable to or greater than those of the graphite/carbon foams. The internal pores of the protective graphene foam expands and shrinks congruently with the expansion and shrinkage of the embedded cathode active material particles, enabling long-term cycling stability of a lithium battery featuring a high-capacity cathode active material (such as FeF3, BiF3, and CuCl2).
It is another object of the present invention to provide a cathode layer that exhibits a combination of exceptional thermal conductivity, electrical conductivity, mechanical strength, and elastic modulus unmatched by any cathode layer commonly used in a lithium-ion battery or lithium metal battery.
Yet another object of the present invention is to provide a graphene foam-protected cathode active material wherein the graphene foam is selected from (a) a pristine graphene foam that contains essentially all carbon only and preferably have a pore size range from 2 nm to 200 nm; or (b) non-pristine graphene foams (graphene fluoride, graphene chloride, nitrogenated graphene, etc.) that contains at least 0.001% by weight (typically from 0.01% to 5% by weight and most typically from 0.01% to 2%) of non-carbon elements.